Even if there were only a single button, like a toggle button--is there such a thing?-- it could then require two terms (labels)... one saying 'on', and one saying 'off'. Both labels could be missing for that button.

Hello! What an interesting question! Maybe some buttons say only "on" because you're not supposed to turn the appliance off? On the other hand, maybe some dangerous appliances that are level with infants' eyes only have the "off" button so they don't turn them on? [:^)]